The present invention relates to an electrode system comprising at least two electrodes formed of an electrode material which comprises molybdenum (Mo) or tungsten (W) or an alloy of molybdenum or tungsten as a main component. The invention in particular relates to a gas discharge electrode system, for example a EUV and/or soft X-ray radiation generating electrode system.
Light sources emitting extreme ultraviolet (EUV) and/or soft X-ray radiation, i.e. in the wavelength region of 1-20 nm, are required for example in the field of EUV lithography or metrology. These devices are by far the most promising candidates to be the high power light source for the upcoming lithography tools of the semiconductor industry. It is known in the art how to create EUV light efficiently at a wavelength of 13.5 nm using a highly ionized plasma. The excitation of such a plasma from an EUV emitting target material may be generated by means of a high power laser beam (laser produced plasma, LPP) or by an electrical gas discharge between electrodes (discharge produced plasma, DPP).
An example of a gas discharge light source is schematically presented and described in connection with FIG. 1. In current state-of-the-art DPP based systems a change from gaseous supply to liquid metal based discharge material in conjunction with two rotating, circular electrodes is used in order to achieve higher conversion efficiencies and thus high amounts of EUV photons required for the target application. A pulsed laser triggers the discharge by evaporating at least partially some of the liquid metal, in particular tin (Sn), from the liquid surface between the electrodes (see e.g. FIG. 2). The capacitor bank is discharged through the tin vapour, creating the high current pinch discharge. There are some decisive features of this type, e.g. the metal melt supply system, the control of the layer thickness of the liquid metal on the electrodes, the necessity to at least partially evaporate the metal melt by means of e.g. a laser beam of some ten mJ pulse energy. There exist many configurations of the application of the metal melt or of the electrode arrangement, for example, however, the use of liquid Sn as a discharge media is generally accepted. Both, in DPP and LPP usually a flow of liquid target material is required to provide the plasma with the EUV emitting species.
Although the scalability to reach high power levels has been fundamentally shown quite recently, one of the potential showstoppers for high volume manufacturing is still the critical requirement of a high power source delivering >200 W at the intermediate focus which corresponds to >1000 W emitted in 2π sr by a point-like light source. In the very near future, the EUV source is expected to run continuously at about 10 kHz pulse frequency and a minimum lifetime of the order of 1011 shots has to be demonstrated. In order to achieve high radiation power, very high average electrical powers have to be fed into the source and this may lead to high electrode wear and thus short lifetimes of the electrode system. For a Xe DPP it is reported that the lifetime of the electrode system is limited by erosion and erosion rates are around 0.2 μg/shot at 11 J/shot for tungsten electrodes which results in significant less EUV output.
Apart from the decrease in lifetime, another disadvantage is due to the fact that the plasma will become spatially larger because of the change in electrode geometry, leading to the effect that only a smaller fraction of the produced light can be exploited and thus, optical performance is diminished. From the statements above it is understandable that dedicated components of the EUV generating device are more or less permanently exposed to relatively harsh physical and chemical conditions at elevated temperatures of greater than e.g. 200° C. A substantial portion of the pulse energy is concentrated in the pinch plasma which leads to thermal loading of the electrodes by the emission of radiation and of hot particles (ions, electrons). Moreover, the pulsed plasma light source is not merely emitting photons, but also producing undesired material, so called debris, such as e.g. Sn, Li, Sb, Xe, . . . depending on the nature of the source itself. In systems with extreme UV or soft X-ray radiation optical components such as mirrors or filters are in use which are close to the high-power light source. The contaminating debris may condense on these optical components whose performance consequently deteriorates and finally becomes inefficacious. To further complicate the situation there is also the prerequisite that the tin needs to be free from contamination (alien elements) in order to secure a high quality of a pure tin plasma. It is well known that pure Sn is much more aggressive than diluted solutions used e.g. in former soldering systems.
In case of the electrode system and its potential problems due to removal of electrode material, there could be several main root causes. The main drivers might be the high heat load and the high current densities along with their spatial and temporal conditions (thermo-mechanical effect). Both are related to the pulsed operation of the plasma and the small distance between the electrodes and the plasma. Due to the high heat fluxes and the high current densities electrode erosion is inevitable. In conjunction with metal melts erosion will lead to an inhomogeneous, thick and unstable layer of liquid metal and thus to a failure in discharge. The minimum thickness of liquid metal is said to be about 5 μm in order to avoid impact of the laser focus spot and cathode spots on the morphology of the electrode surface (e.g. crater formation). Too thick a layer of metal melt (beyond about 40 μm) is also disadvantageous in controlling the amount of debris material in the pinch plasma. It is also stated that erosion mechanisms may include volumetric boiling and explosion of large bubbles developed in the surface layer (of e.g. W with dissolved gases) and even splashing of the molten electrode material at the surface due surface wave excitation caused by energetic particles from the plasma.
Furthermore, another cause for the removal of electrode material can be found in the chemical corrosion due to liquid Sn at elevated temperatures (thermo-chemical effect). As this is a thermo-dynamically driven process it is more pronounced at an increased temperature level. This is not only detrimental to the electrode system by erosion, but also may reduce the EUV performance, i.e. output of EUV light, as unwished elements or reaction products which do not emit efficiently in the EUV wavelength range may become part of the discharge region. It has been shown that in particular in case of pure Sn the liquid is very aggressive to common materials such as stainless steel and gray cast irons.
WO 2005/025280 A2 discloses a gas discharge light source emitting EUV and/or soft X-ray radiation. The electrodes of this light source are disclosed to be made of Cu, W or Cu covered with Mo. These materials are highly heat conductive. Moreover, between the electrodes and the liquid metal a high electrical conductivity is also required. In case of (solid) Sn as a target material the values of electrical resistivity and thermal conductivity are 11 μΩcm and 67 W/(mK), respectively, both worse than the values of common electrode materials. Thus, the thinner the Sn layer can be made the more heat can be dissipated and the more electrical power can be coupled in. Generally, W and Mo are expected to be promising refractory transition metals for use in extreme environments. However, they are known to undergo serious embrittlement in several regimes, e.g. recrystallization embrittlement, which inevitably limits structural application of W and Mo. The weakness of the grain boundaries stems from their intrinsic character and recrystallization and embrittlement are further potential failure modes at elevated temperatures.
In case of solid electrodes, i.e. Xe DPP, a method is known to reduce electrode erosion by treating the surface of the electrode to allow for increased re-deposition of eroded electrode material into fabricated grooves. Alternatively, coatings by a porous material to reduce erosion due to brittle destruction is also proposed. In another known technique, dedicated sputter deposition is mentioned to regenerate the electrodes by material and account for the loss.